Generally, a manufacturing process for a semiconductor device such as an LSI or VLSI formed from a micropattern uses a reduction type projection exposure apparatus which reduces and projects a circuit pattern drawn on a mask onto a substrate coated with a photosensitive agent and exposes it. With an increase in the packaging density of semiconductor devices, demands have arisen for further micropatterning. Exposure apparatuses are coping with micropatterning along with the development of a resist process.
An ArF excimer laser with an oscillation wavelength around far infrared rays, particularly, 193 nm, and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm are known to have absorption bands for oxygen (O2) and moisture (H2O). Therefore, in the optical path of the exposure optical system of a projection exposure apparatus using a far ultraviolet laser such as an ArF excimer laser or a fluorine (F2) excimer laser as a light source, the oxygen and moisture concentration (to be referred to as an impurity concentration hereinafter) present in the optical path must be suppressed to a low level of ppm order or less by a purge means using inert gas such as nitrogen.
In such an exposure apparatus using the far ultraviolet laser as the light source, the exposure path is partly purged with inert gas. For example, FIG. 1 is a view for explaining a purge means in the vicinity of a wafer shown in Japanese Patent Laid-Open No. 2001-358056.
In the exposure apparatus portion shown in FIG. 1, a wafer stage 110 supported by an installed surface plate through an anti-vibration unit is mounted on a stage surface plate 118. A frame 117 supports a projection optical system 105. The frame 117, the stage surface plate 118, and partition walls 113 and 114 form an isolated space including the wafer stage 110.
FIG. 1 shows a purge means as follows. A cover 109 extends from the wafer-side lower end of the projection optical system 105 toward the vicinity of the wafer stage 110 to surround an exposure optical path. The cover 109 has a supply port 106 through which purge gas formed of inert gas blows out. A recovery port 107 through which the purge gas is drawn by suction is formed to oppose the supply port 106. Thus, the purge gas is supplied to flow inside the cover 109. In FIG. 1, a gap is formed between the cover 109 and wafer 111 so that the cover 109 and a wafer 111 will not interfere with each other. Various types of products (released gas) generated during exposure from a photosensitive agent applied on the substrate surface may attach to the surface of an optical element to decrease the efficiency of the optical system. In view of this, the purge gas flows in one direction (indicated by 108) inside the cover 109, so that the released gas can be recovered efficiently.
Around the wafer stage, the temperature and humidity must be maintained at predetermined values. Hence, generally, temperature- and humidity-adjusted air, or temperature- and humidity-adjusted inert gas having a higher impurity concentration than that of the purge gas to be supplied to the inside of the cover 109, is supplied to flow around the wafer stage and projection lens.
As described above, in the exposure apparatus utilizing far ultraviolet rays, the cover 109 is formed at the wafer-side end of the projection optical system 105. The purge gas supply port 106 and exhaust port 107 are formed inside the cover 109. The purge gas is supplied to flow in one direction, thus performing purging. According to the present inventor, however, since the gap is formed between the cover 109 and wafer 111, a whirl tends to be generated in the vicinity of the lower side of the supply port 106 (see FIG. 2A). Where the whirl is generated, the pressure is lower than the pressure around the cover 109. Accordingly, ambient air or purge inert gas having a high impurity concentration enters the inside of the cover 109. This degrades the impurity concentration inside the cover 109 to undesirably decrease the transmittance of the exposure light. As a result, the exposure time prolongs, and the productivity of the apparatus is degraded.
A state wherein a whirl is generated in the vicinity of the lower side of the supply port 106 to decrease the pressure will be described with reference to FIGS. 2A and 2B. FIG. 2A is a stream diagram of the purge gas in the central section inside the cover 109, and FIG. 2B is a schematic contour diagram of the pressure distribution taken at the same location as that of FIG. 2A. In the schematic contour diagram of FIG. 2B, the darker the color, the higher the pressure; the lighter the color, the lower the pressure.
When a gap is formed between the cover 109 and wafer 111, a whirl is generated in the vicinity of the lower side of the supply port 106, as shown in FIG. 2A. Where the whirl is generated, the pressure is low, as shown in FIG. 2B. When a pressure P1 in the vicinity of the lower side of the supply port 106 becomes lower than a pressure P2 outside the cover 109, air or purge inert gas with a high impurity concentration enters the cover 109 from around. Hence, to purge the inside of the cover 109 stably, the pressure inside the cover 109, particularly the pressure P1 in the vicinity of the lower side of the supply port 106, must be kept higher than the pressure P2 around the cover 109.
According to the present inventor, the flow (to be referred to as ambient flow hereinafter) of air and purge inert gas around the projection lens and wafer stage outside the cover 109 also largely adversely affects the purging performance inside the cover 109. The gas flow and pressure distribution in the vicinity of the cover 109, when an ambient flow exists, will be described with reference to FIGS. 3A and 3B.
FIG. 3A is a stream diagram of the flow of the inert gas around the cover 109, and FIG. 3B is a schematic contour diagram of the pressure distribution. In the schematic contour diagram of FIG. 3B, the darker the color, the higher the pressure; the lighter the color, the lower the pressure. The cover 109 serves as an obstacle against an ambient flow 119. Therefore, as shown in FIG. 3A, the ambient flow 119 collides against the cover 109 and changes its course. At this time, as shown in FIG. 3B, around the cover 109, the pressure against the ambient flow 119 is the highest on the upstream of the cover 109. Accordingly, ambient air or purge inert gas with a high impurity concentration tends to enter the inside of the cover 109 from the upstream of the ambient flow 119.
When an ambient flow is present in this manner, the inside of the cover 109 must be purged while considering the pressure distribution in the vicinity of the cover 109 as well. It is not easy to purge the inside of the cover 109 stably without being adversely affected by the ambient flow. Although FIGS. 3A and 3B exemplify a case wherein the cover 109 has a rectangular shape, the same applies to a case wherein the cover 109 has a circular cylindrical shape or the like.
In view of the above situation, demands have arisen for a purge means that can increase the pressure inside the cover 109, particularly the pressure in the vicinity of the lower side of the supply port 106 formed inside the cover 109, to be higher than the pressure outside the cover 109, and a means that can stably purge the inside of the cover 109, even if the ambient flow 119 is present, without being adversely affected by the ambient flow 119.
Generally, the photonic energy of exposure light increases in inverse proportion to the wavelength of exposure light. Particularly, the photonic energy of a fluoride (F2) excimer laser having an oscillation wavelength around 157 nm is as very large as 7.9 eV, and can disconnect molecule bonds constituting most resins. For this reason, when a resin-based photosensitive agent applied to the wafer is irradiated with exposure light, moisture adsorbed by the surface layer of the photosensitive agent evaporates, and part of the photosensitive agent decomposes and is highly likely released as impurities together with the moisture adsorbed by the surface layer of the photosensitive agent.
The impurities (to be referred to as released gas hereinafter) generated during exposure by the photosensitive agent itself applied to the wafer and by the moisture adsorbed by the surface of the photosensitive agent are recovered by the one-directional flow of the purge gas inside the cover 109. Due to the flow of the purge gas, the released gas forms a concentration gradient inside the cover 109. As the exposure light is absorbed by the released gas, as well, a transmittance loss distribution is formed in the exposure area due to the concentration gradient of the released gas.
The concentration distribution of the released gas, which occurs inside the cover 109, and the transmittance loss distribution in the exposure area will be described with reference to FIGS. 4A to 4C. FIGS. 4A and 4B are schematic contour diagrams of the concentration distribution of the released gas which is formed when gas release occurs during exposure. In the schematic contour diagram of FIG. 4A as a central sectional view of the inside of the cover 109 and that of FIG. 4B as a plan view, the darker the color, the higher the concentration of the released gas; the lighter the color, the lower the concentration of the released gas. As shown in FIGS. 4A and 4B, when gas release occurs during exposure, the concentration of the released gas is high and low on the downstream and upstream, respectively, of a purge gas flow 108 in the exposure area, thus forming a concentration gradient.
FIG. 4C is a schematic contour diagram of the transmittance loss in the exposure area when the concentration gradient shown in FIGS. 4A and 4B occurs. In the schematic contour diagram of FIG. 4C, the darker the color, the larger the transmittance loss; the lighter the color, the smaller the transmittance loss. As shown in FIG. 4C, when the released gas forms a concentration gradient during exposure, in the exposure area, the transmittance loss is small and large on the upstream and downstream, respectively, of the purge gas flow 108. Therefore, the exposure amount obtained in the exposure area during exposure forms a distribution, thus causing illuminance nonuniformity.
In this manner, when gas release occurs, illuminance nonuniformity occurs in the exposure area. A desired exposure amount may not be obtained in part of the exposure area, and the circuit pattern of the mask cannot be exposed sufficiently. As a result, the manufactured semiconductor devices include many defective products, and the productivity of the apparatus degrades. In view of this, demands have arisen for development of a purge means that does not cause illuminance nonuniformity during exposure even when a one-directional flow is present inside the cover 109.